British English refers to the dialects of the English language spoken in the United Kingdom. A popular sense of the term is that it means only the 'standard', typically written form of English in the UK and especially southern England. Linguists, however, would use it to mean any dialect, standard or not, that is used in England, Scotland, Wales and in Northern Ireland, plus the territories surrounding the UK. Often the accents of the UK are included in the definition too.
One reason that 'British English' might be associated with standard written language is that one of the most obvious differences between this and other varieties of English is its spelling conventions: colour rather than American English color, for example. These so-called 'British' spellings are however common in many other countries, generally those that were part of the old British Empire. Today, one aspect of culture shared between the Commonwealth of Nations countries is many of the spellings that predominantly originated in Britain: these are part of what is sometimes called 'Commonwealth English'. This term does, however, exclude any country not a member of the Commonwealth, such as the Republic of Ireland, where 'British' spellings are part of Hiberno-English.
Varieties of British English
British English is not a single dialect, similar for all speakers throughout the UK and the Commonwealth. It differs from region to region and often extensively within the same towns, cities and local communities. For example, while Cockney is the form of English most strongly associated with the East End of London, other varieties have made inroads. What linguists have called Multicultural London English - to others, 'Jafaikan' - is now often found in the speech of young Londoners, blending pronunciations and grammar from such varieties as British Jamaican and Bangladeshi English, as well as others forms drawn from the speech of historically immigrant communities.
While some might use this to refer to standard English or the prestige accent known as Received Pronunciation (RP), to linguists it refers to all the varieties of English spoken in England - again, whether standard or not.
Scottish English is another umbrella term, this time for all the varieties of English known in Scotland. There is some dispute, however, over whether Scots should be identified with English or referred to as a language in its own right. Certainly, the Scots of Glasgow contrasts sharply in grammar and pronunciation with 'Standard Scottish English', whose written and spoken conventions are similar to other standard forms of English, albeit with a distinct Scottish accent.
Wales, like the rest of the UK, is home to a rich variety of accents and dialects - these often influenced by the Welsh language, even where Welsh is not widely spoken.
Northern Irish English
Though the standard forms of British and Irish English (or Hiberno-English) are very similar, the history of Northern Ireland makes identifying the varieties of English spoken there with the UK slightly controversial. Politically and culturally, while many Northern Irish speakers may refer to their overall dialect as one variety of British English, others would prefer to describe it as a form of Irish English. Therefore, whether Northern Irish English is also British English or Irish English is a matter of political perspective. Linguistically, the dialects of Northern Ireland share roots with both the English of elsewhere on the island of Ireland and those spoken on the island of Great Britain.
Specifically in Northern Ireland, English usage is influenced by the Irish language (a Celtic language only distantly related to English) and Scots (a Germanic variety sometimes over-simplistically labelled an English dialect). Groupings can be identified as, for example, Hiberno-English, Ulster-Scots and/or Ullans (a portmanteau of the words Ulster and Lallans). Which variant of a dialect in usage is often, but not always, determined by region. The Northern Irish sometimes use English words from, for example, the Victorian era, that are considered archaic elsewhere.
The wider world
The spelling conventions of English varieties in many other countries are drawn from British English - standard written Jamaican English, for example, is difficult to distinguish from the British form. Likewise, countries where English has been adopted as an official or recognised language make use of British or 'Commonwealth' English spellings, as in standard Singapore English, for example.
Is 'British English' also 'Standard English'?
'Standard British English' - meaning the formal variety of UK English which is the subject of dictionaries and grammar books, and is taught in schools and to learners of English in the UK and in various places overseas - is just one form of British English today. Linguists would consider its study highly valuable, but other dialects would be equally worthy of consideration. Confining the definition of 'British English' to the standard variety also creates a problem: all forms of a living language change all the time, as new generations develop new vocabulary or reanalyse one aspect of their native language's grammar in a slightly different way from how their parents used it. This also applies to the 'standard' language, though this process is typically slowed by its being codified in a set of written conventions and prescriptivist grammar rules. If linguists were to agree with the popular definition of British English as standard English, new innovations would go uncatalogued and linguistic diversity could not be used as data to further understanding of language itself.